1. Technical Field
The present invention relates to liquefied gas storage tanks and in one aspect relates to a tank especially adapted for storing cryogenic liquefied gases (e.g., liquefied natural gas (xe2x80x9cLNGxe2x80x9d)) at cryogenic temperatures at near atmospheric pressures in areas susceptible to earthquake activity.
2. Background
Liquefied natural gas (LNG) is typically stored in double walled tanks or containers. The inner tank provides the primary containment for the LNG while the outer shell holds the insulation in place and protects the inner tank and the insulation from the adverse effects of the environment. Sometimes, the outer tank is also designed to provide a secondary containment of LNG and associated gas vapor in case the inner tank fails. Typical sizes of onshore tanks at import or export terminals range from 50,000 to 100,000 cubic meters although tanks as large as 200,000 cubic meters have been built or are under construction.
Two distinct types of tank construction are widely used for storing LNG at onshore locations. The first of these comprise a flat-bottomed, cylindrical, self-standing tank which typically uses a 9% nickel steel for the inner tank and carbon steel, 9% nickel steel, or reinforced/prestressed concrete for the outer shell. The second type is a membrane tank wherein a thin (e.g. 1.2 mm thick) metallic membrane is installed within a cylindrical concrete structure which, in turn, is built either below or above grade on the ground. A layer of insulation is interposed between the stainless steel or Invar membrane and the load bearing concrete cylindrical walls and flat floor.
Recently, radical changes have been proposed in the construction of LNG terminals, especially import terminals. One such proposal involves the building of the terminal a short distance offshore where the LNG will be off-loaded from a transport vessel, stored, retrieved, and regasified before it is piped to shore for sale or use. Possibly one of the more promising of this type of terminals is one where the LNG storage tanks and regasification equipment will be installed on gravity base, box-shaped, barge-like structures (GBS) similar to certain concrete gravity structures now installed on the seafloor and being used as platforms for producing petroleum in the Gulf of Mexico.
Unfortunately, neither cylindrical tanks nor membrane tanks are considered as being particularly attractive for use in storing LNG on GBS terminals. Cylindrical tanks take up too much room on the GBS in relation to the volume of LNG which can be stored therein and are difficult and expensive to construct on such. Further the size of such tanks must be limited (e.g. 50,000 cubic meters) so that the GBS structures can be fabricated economically with readily available fabrication facilities. This necessitates a multiplicity of storage units to satisfy particular storage requirements which is not desirable from cost and operational safety considerations.
A membrane-type tank system, on the other hand, can be built inside the GBS to provide a relatively large storage volume. However, a membrane-type tank requires a sequential construction schedule wherein the outer concrete structure has to be completely built before the insulation and the membrane can be installed within a cavity within the outer structure. This normally requires a long construction period which adds substantially to the costs. Further, membrane-type tanks are designed by principles known as xe2x80x9cexperimental designxe2x80x9d wherein the guarantee of satisfactory performance of a particular tank and its safety are based on historical experience and laboratory studies rather than on rigorous demonstration by analysis and quantified experience. Where new shapes and sizes are required or when different environmental and/or seismic loading conditions are to be encountered, the satisfactory performance of membrane-type tanks at various LNG levels is difficult to insure.
Accordingly, a tank system is needed for near offshore storage of LNG which alleviates the above-discussed disadvantages of both cylindrical tanks and membrane-type tanks. Such a tank is a polygonal-shaped, box-like, structure which can be fitted into a space within a steel or concrete GBS and which is capable of storing large volumes (e.g. 100,000 cubic meters and larger) of LNG at cryogenic temperatures. The tank should also perform safely at various LNG levels in areas where seismic activity (e.g. earthquakes) is encountered and where such activity may induce liquid sloshing and associated dynamic loads within the tank.
Similar box-shaped, polygonal tanks have been used for storing LNG aboard sea-going, transport vessels. One such tank, popularly known as the xe2x80x9cConchxe2x80x9d tank, (e.g. see U.S Pat. No. 2,982,441) has been built from 9% nickel steel or aluminum alloys. In its original design as proposed by the above referenced patent, the tank is constructed of six plate panels (i.e. the four sides, the top or roof, and the bottom or floor of the tank) which are reinforced or xe2x80x9cstiffenedxe2x80x9d only by horizontal beams and stiffeners or the like. According to the inventors, vertical stiffening is deliberately omitted in order to eliminate or reduce thermal stresses due to thermal gradients in the vertical direction as the volume of LNG in tank changes.
In the xe2x80x9cConchxe2x80x9d tank, horizontal tie rods may be provided (a) at the corners at the vertical interfaces of the walls to strengthen the corners; and/or (b) as connections between the opposite faces of the walls to lessen the panels deflections. Nonetheless, horizontally-stiffened wall panels and two-way stiffened floor and roof plate panels, as embodied in the above referenced patent, basically provide the structural strength and stability for the tank. The original tanks built with this concept are reported to be less than 10,000 cubic meter in capacity.
When the Conch design (as illustrated in U.S. Pat. No. 2,982,441) is extended to larger tanks, a design similar to FIG. 1 can be expected (i.e. a known, prior-art, prismatic tank developed by IHI Co., Inc. of Tokyo, Japan). Modern materials and design methods do not restrict provision of vertical stiffening by consideration of thermal gradient as the liquid level of LNG changes. Consequently, the illustrated prismatic tank consists of wall plate panels that are stiffened by both horizontal and vertical beams/stiffeners. But even for a relatively small size of 23,500 cubic meters, to achieve satisfactory strength and stiffness during construction handling and operational use, the xe2x80x9cConchxe2x80x9d tank must be provided with intermediate stiffened panel bulkheads and diaphragms, as illustrated by a vertical bulkhead in each of the length and width directions of the IHI tank. This type of design is believed to be good only for tanks having a relatively small storage capacity.
A larger tank suitable for use on a modern terminal and designed in accordance with the prior art would need still more bulkheads to support the roof structure and to provide structural strength and stability of the tank in operational use (e.g. see FIG. 2). Accordingly, a typical large storage tank might in effect be considered as consisting of several of the smaller Conch-type tanks aligned wherein a common wall between adjacent tanks forms a horizontal or transverse bulkhead within the overall storage volume of the complete storage system.
For applications on ships and other transport vessels, the bulkheads within the tanks not only provide strength and stability for a relatively large, storage tank but also reduce the dynamic loads on the tank due to any sloshing of the LNG within the tank caused by movement of the floating vessel during transport. The dynamic excitation of the storage tank due to the oscillatory motion of the ship caused by wind and wave action, has relatively large periods (e.g. 6-12 seconds). Fundamental periods of liquid sloshing within small cells created by bulkheads within the tank are relatively small thus avoiding resonance and amplification of sloshing loads. While the bulkhead construction makes such tanks suited for the marine transportation of LNG, it has certain drawbacks when applied to onshore or bottom-supported storage (e.g. GBS), primarily because in these environments, the dynamic excitation caused by seismic activity (e.g. earthquakes, etc.) is of much shorter periods (e.g. xc2xd to 1 second).
Due to the closeness of the fundamental periods of sloshing waves in small constrained spaces and the predominantly xe2x80x9cshortxe2x80x9d excitation periods caused by seismic activity, the relative xe2x80x9cshortxe2x80x9d dimensions of the individual compartments formed by the bulkheads in a storage tank become highly detrimental when sloshing in the tank occurs due to seismic activity. Accordingly, it is desirable for the storage space within a land-based LNG tank or a tank installed on a GBS which, in turn, is installed on the sea bottom, to be long and unimpeded since such open space helps to reduce the dynamic loads caused by the shorter excitation periods which will be encountered should any seismic, activity occur. Further, the large number of compartments, which are typically formed within the tank by the bulkheads, require multiple cryogenic pumping and handling systems for filling and emptying the tank and multiple penetrations and connections through the roof, which, in turn, lead to increased capital and operating costs as well as increasing the safety hazards normally involved with the storage and handling of LNG.
The present invention provides a large, box-like polygonal tank for storing liquefied gas which is especially adapted for use on land or in combination with bottom-supported offshore structure such as gravity-based structures (GBS) and a method of constructing the tank. xe2x80x9cBox-like tankxe2x80x9d, as used herein, is intended to refer to a polygonal tank having two end walls, side walls, a top, and a bottom. Basically, the tank is comprised of (a) an internal, two-way truss frame structure, i.e., trusses in vertical planes, aligned in and crises-crossing along longitudinal (i.e., along the length) and transverse (i.e., along the width directions) and (b) a cover, sealingly enclosing the frame, for containing the stored liquid within the tank.
The internal, truss-frame is comprised of a plurality of vertical, elongated supports and horizontal, elongated supports, connected at their respective ends to form a box-like frame which, in turn, has tubular and non-tubular beams, column and brace members secured therein to provide additional strength and stability along the length and width directions of the truss frame. A plurality of stiffened or unstiffened plates (e.g. 9% nickel-steel, aluminum, aluminum alloys, etc.) are secured to the outside of box-like frame to form the cover for the tank.
Many different arrangements of the beams, columns and braces can be devised to achieve the desired strength and stiffness of a truss frame as illustrated by the use of trusses on bridges and other civil structures. For the tank of the present invention, the truss frame construction in the longitudinal and transverse directions may not be identical, or even similar. Rather, the trusses in the two directions are designed to provide the specific strength and stiffness required for the over all dynamic loads caused by seismic activity, the need to support the large roof structure and the loads due to the unavoidable unevenness of the floor. In the preferred embodiment of this invention, suitable for areas of moderate seismic activity, the internal truss structure may be provided only in the transverse direction with no truss(es) in the longitudinal direction.
More specifically, the large, box-like polygonal storage tank of the preferred embodiment of the present invention is comprised of two substantially identical end sections and none, one, or a plurality of intermediate sections. All of the intermediate sections have basically the same construction and each is comprised of a rigid frame which, in turn, is formed of at least two vertical, elongated supports and at least two horizontal, elongated supports, connected at their respective ends. Additional supports, beams, columns and brace members are secured within said frame to provide additional strength and stability to the frame. A plurality of plates are secured to the outside of said frame which form the cover or containment walls of said tank when the respective sections are assembled.
By using a box-like internal truss frame to provide the primary support for the tank, the interior of the tank will be effectively contiguous throughout without any encumbrances provided by any bulkheads or the like. This permits the relatively long interior of the present tank to avoid resonance conditions during sloshing under the substantially different dynamic loading caused by seismic activity as opposed to the loading which occurs due to the motion of a sea-going vessel.